India is running out of phosphorus; does the answer lie in our sewage?

Contact Counsellor

Last Updated28/09/2023

The problem with the fertilisation of land is as old as agriculture itself.
When early humans first began to engage in settled agriculture, they quickly realised that while crops require nutrients for their growth, repeated cycles of cultivation and harvest depleted these nutrients, reducing yield over time.

This observation led to practices to restore essential nutrients in the soil necessary for plant and crop growth.
Indigenous communities around the world developed methods of fertilisation, for example, using fish remnants and bird droppings (guano) as fertilisers.
This changed in the 19th century, which saw significant advancements in chemistry, leading to the creation of synthetic fertilisers as well as the identification of nitrogen, phosphorus, and potassium.
The Green Revolution of the mid-20th century accelerated the adoption of high-yield crop varieties and intensive use of these fertilisers, and today these substances are crucial to sustain global food production.

Phosphorus is scarce and exists only in limited quantities, in certain geological formations.
Not only are we running out of it, it also pollutes the environment.
It doesn’t exist as a gas, which means it can only move from land to water, where it leads to algal blooms and eutrophication.

The history of phosphorus spans its discovery in guano to current global supply chains.
The world’s largest reserves are in Morocco and the Western Sahara region.
But here, phosphorus coexists with cadmium, a heavy metal that can accumulate in animal and human kidneys when ingested.
Removing cadmium is also an expensive process.
As a result, cadmium-laden fertilisers are often applied to the soil, absorbed by crops, and consumed, bioaccumulating in our bodies.
Studies have found that this accelerates heart disease.
Only six countries have substantial cadmium-free phosphorous reserves.
Of them, China restricted exports in 2020 and many EU countries no longer buy from Russia. So the market for safe phosphorus has suddenly exploded.
This is one reason why Sri Lanka banned the import of synthetic fertilisers and went organic in 2021, later experiencing a sudden drop in crop yield that precipitated a political crisis.
Today, India is the world’s largest importer of phosphorus, most of it from the cadmium-laden deposits of West Africa.
Not all crops absorb cadmium at the same rate, but paddy, a staple crop in India, is particularly susceptible; Indian farmers also apply a lot of fertilisers to paddy.
Other grains, such as wheat, barley, and maize also absorb cadmium, just less.

First, only about a fifth of the phosphorus mined is actually consumed through food. Much of it is lost directly to water bodies as agricultural run-off, due to the excessive application of fertilisers.
Second, most of the phosphorus that people consume ends up in the sewage. Most sewage in India is still not treated or treated only up to the secondary level.
So even if the organic matter is digested, the effluent discharged from STPs still contains nitrates and phosphates.
Of these, nitrates can be digested by denitrifying bacteria and released safely as nitrogen gas into the atmosphere, while phosphorus remains trapped in the sediments and water column.
It is then absorbed by the algal blooms that grow in response to the high nutrient supply, and when they decompose, the bacteria that feed on them consume the dissolved oxygen.
The result: water bodies become oxygen-starved, leading to fish deaths. The algal blooms are also toxic, causing respiratory issues, nausea, and other ailments to people exposed to them.

Since much of the phosphorus is not actually taken up by crops, one way to ameliorate the phosphorus paucity is to reduce the use of chemical fertilisers through precision agriculture.
Low-input agro-ecological approaches are increasingly proving to be a viable alternative.
But there is increasing interest in closing the phosphorous loop by mining urban sewage to produce high quality phosphorus.
Interest in ‘circular water economies’ has in fact prompted the European Union – which has almost no phosphorus reserves of its own – to rethink the urban water cycle.
First, source separating toilets – almost two thirds of the phosphorus we consume leaves in our urine and the rest in faeces.
Urine also contains large amounts of nitrogen and potassium. If we can collect this safe and concentrated waste stream, we could generate a local fertiliser source.
Second, recycling wastewater and sludge – Sewage recycling already occurs in some form in India today.

The best way is to create a circular water economy. If the technology is cheap enough, can we give a concession to set up STPs with phosphorus mining plants and allow them to sell the fertiliser.
And such changes, India can become less dependent on uncertain geopolitical crises; farmers can procure fertilisers at affordable rates; water bodies will have some hope of becoming swimmable and public health can gain from the consumption of food grown in cadmium-free soils.